1. Technical Field
The present invention relates in general to power supplies and, in particular, to a battery pack having a number of individual series-connected cells that combine to produce a particular supply voltage. More specifically, the present invention relates to an active bypass circuit for extending energy capacity and operational life of a multi-cell battery.
2. Description of the Related Art
A battery is typically an electrochemical device that converts chemical energy into electrical current utilizing one or more galvanic cells. A galvanic cell is a fairly simple device that consists of two electrodes, i.e., an anode and a cathode, typically immersed in an electrolyte solution. The amount of energy, i.e., voltage and current, that a galvanic cell generates is directly related to the types of materials employed in the electrodes and electrolyte. The length of time that the cell can maintain a particular voltage and current is related to the amount of active material utilized in the cell and the cell's design.
A key parameter for a battery cell is its capacity, generally stated in ampere hours, or milliampere hours. The capacity of the battery cell is a variable that changes depending on discharge rate, charge rate, temperature, age and number of charge/discharge hours. Cell voltage also varies as a function of temperature, discharge rate and discharge status, i.e., percent of discharge.
A battery cell's voltage profile is the relationship of its output voltage to the amount of time that the battery cell has been discharging, e.g., when connected to a load device, or charging. Generally, in most battery cells, the voltage will reduce steadily as the chemical reactions in the cell are diminished. In the case of nickel-cadmium (Ni--Cd) cells, the output voltage is a relatively flat voltage profile. A Ni--Cd cell's voltages will typically remain constant over approximately two-thirds of the cell's discharge cycle. At some point near the end of the cell's discharge cycle, the cell's voltage will drop sharply to nearly zero volts. This requires that the cell will have to be replaced, or recharged, almost immediately after a drop in voltage. If the battery cell is not replaced, or charged, immediately, the cell will quickly cease to provide any useful energy. Additionally, a cell's voltage that has dropped to zero and is continuing to be used may result in the failure in the cell itself.
In either event, a voltage drop, due to a discharged cell or a failure in the cell, may also adversely impact the rest of the cells in multi-cell battery, contributing to additional failures. Additionally, current energy systems utilizing multi-cell battery packs consider the battery pack as the lowest level field replaceable unit. Thus a failure in one or more (but not all) of the cells in the battery pack will necessitate a replacement of the entire battery pack even if the remaining "good" cells may still have sufficient energy capacity to continue providing power to a load.
Accordingly, what is needed in the art is an improved multi-cell battery system that mitigates the above-discussed limitations in the prior art. More specifically, what is needed in the art is a multi-cell battery system with improved reliability and longer life cycle.